This invention relates generally to the field of film thickness measurement, and more specifically, to the field of film measurement in an environment, such as semiconductor wafer fabrication and processing, in which the layer whose thickness is desired to be measured resides on a patterned sample.
Many industrial processes require precise control of film thickness. In semiconductor processing, for example, a semiconductor wafer is fabricated in which one or more layers of material from the group comprising metals, metal oxides, insulators, silicon dioxide (SiO2), silicon nitride (SiN), polysilicon or the like, are stacked on top of one another over a substrate, made of a material such as silicon. Often, these layers are added through a process known as chemical vapor deposition (CVD), or removed by etching or removed by polishing through a process known as chemical mechanical polishing (CMP). The level of precision which is required can range from 0.0001 μm (less than an atom thick) to 0.1 μm (hundreds of atoms thick).
To determine the accuracy of these processes after they occur, or to determine the amount of material to be added or removed by each process, it is advantageous to measure the thickness of the layers that are on each product wafer (i.e., on each wafer produced that contains saleable product), which is generally patterned with features on the order of 0.1 μm to 10 μm wide. Because the areas covered by these features are generally unsuitable for measurement of film properties, specific measurement “pads” are provided at various locations on the wafer. To minimize the area on the wafer that is taken up by these measurement pads, they are made to be very small, usually about 100 μm by 100 μm square. This small pad size presents a challenge for the film measurement equipment, both in measurement spot size and in locating the measurement pads on the large patterned wafer.
To date, though its desirable effects on product yield and throughput are widely recognized, thickness measurements are only made after certain critical process steps, and then generally only on a small percentage of wafers. This is because current systems that measure thickness on patterned wafers are slow, complex, expensive, and require substantial space in the semiconductor fabrication cleanroom.
The most widely used technique for measuring thin-film thickness on both patterned and unpatterned semiconductor wafers is spectral reflectance. Conventional systems for measuring thickness on patterned wafers employ high-magnification microscope optics along with pattern recognition software and mechanical translation equipment to find and measure the spectral reflectance at predetermined measurement pad locations. Examples of this type of system are those manufactured by Nanometrics, Inc., and KLA-Tencor. Such systems are too slow to be used concurrently with semiconductor processing, so the rate of semiconductor processing must be slowed down to permit film monitoring. The result is a reduced throughput of semiconductor processing.
A newer method for measuring thickness of patterned films is described in U.S. Pat. No. 5,436,725. This method uses a CCD camera to image the spectral reflectance of a full patterned wafer by sequentially illuminating the wafer with different wavelengths of monochromatic light. Because the resolution and speed of available CCD imagers are limited, higher magnification sub-images of the wafer are required to resolve the measurement pads. These additional sub-images require more time to acquire and also require complex moving lens systems and mechanical translation equipment. The result is questionable advantage in speed and performance over traditional microscope/pattern recognition-based spectral reflectance systems.
Accordingly, it is an object of the present invention to provide a method and apparatus for achieving rapid measurement of film thickness and other properties on patterned wafers during, between, or after semiconductor processing steps.
An additional object is a method and apparatus for film measurement which is capable of providing an accurate measurement of film thickness and other properties of individual films in a multi-layered or patterned sample.
A further object is an optical method and apparatus for thin-film measurement which overcomes the disadvantages of the prior art.
Further objects of the subject invention include utilization or achievement of the foregoing objects, alone or in combination. Additional objects and advantages will be set forth in the description which follows, or will be apparent to those of ordinary skill in the art who practice the invention.